1. Field of the Invention
The present invention relates to a brake apparatus for the pressure supply of the brakes of a hydraulic two-circuit brake installation of a road vehicle, in which at least one brake circuit is constructed as static brake circuit which is connected to an outlet pressure space, in which the brake pressure build up takes place by unidirectional displacement of the servo piston and of a master cylinder piston, whereby the servo piston experiences a pressure build-up displacement only when the brake servo unit operates properly and whereby the master cylinder piston experiences a pressure build-up displacement when the brake force servo unit is intact, as also when the brake force servo unit has failed, whereby in that case, in relation to a predetermined amount of the residual actuating force engaging at the master cylinder piston, an increase of the piston displacement travel and therewith a correlated pressure translation occurs.
2. Description of the Prior Art
Such a brake apparatus is known from the DE 34 44 828 A1.
In the known brake apparatus, a tandem master cylinder is coordinated to the static brake circuit of the primary outlet pressure space, along the central axis of which a linear hydrocylinder is arranged separated from the primary outlet pressure space by a partition wall, whose piston is provided on the side of the master cylinder with a plunger which passes through a central bore of the partition wall, sealed off with respect to the same, and is axially supported at the primary piston of the master cylinder.
The actuating cylinder piston has a piston rod on the side of the pedal which passes through an end wall of the cylinder housing also displaceably sealed pressure-tight with respect to the same. The pedal plunger is supported at this piston rod, by way of which the force exerted by the driver is introduced into the piston of the actuating cylinder. A brake valve is integrated into this piston which, depending on the actuating direction, couples a pressure proportional to the pedal force exerted by the driver, which pressure is derived from the high outlet pressure of an auxiliary pressure source, either into the actuating pressure space delimited axially in a fixed manner by the end wall of the cylinder housing and movably by the piston or--in case of a retraction movement of the brake pedal--into the space which is delimited axially movably by the piston of the actuating cylinder and fixedly by the partition wall which delimits the primary outlet pressure space of the master cylinder against this space of the actuating cylinder.
This actuating piston and together with the same the primary piston of the master cylinder is displaceable in the sense of a brake pressure build-up both when the brake servo unit is intact as also when the brake servo unit fails. The high pressure inlet chamber of the actuating cylinder is thereby constructed as annular space which is delimited in the axial direction by flanges of the actuating piston which are arranged at a distance from one another and which are sealed off with respect to the cylinder bore by O-rings.
A second housing bore is arranged adjacent the master cylinder and the actuating cylinder, having an arrangement parallel to the common longitudinal axis thereof, whereby a stepped piston is displaceably guided pressure-tight in the second housing bore, whose smaller piston step axially movably delimits an outlet pressure space in communicating connection with the primary outlet pressure space of the master cylinder, and whose larger piston step, together with a partition wall of the cylinder housing delimits a further actuating pressure space which is operatively connected by way of a function-control valve with the one actuating pressure space of the actuating cylinder which is fixedly delimited by the end wall of the cylinder housing on the pedal side. This second servo piston is displaceable in the sense of a brake pressure build-up within the outlet pressure space delimited by its smaller piston step and communicating with the primary outlet pressure space, only when the brake force servo unit is intact. This second booster or servo piston is provided with an L-shaped lug which axially engages also at the actuating piston containing the brake valve but is not securely connected with the same. As a result thereof, the actuating piston containing the brake valve is "taken along" by the second servo piston, so to speak of, when the brake force servo unit is operable; however, the second actuating piston remains stationary in its base position, into which it is urged by a return spring, when the brake force servo unit is not operable.
The considerable disadvantage of the known brake apparatus resides in that in case of a failure of the brake force booster or servo unit, disproportionately high pedal-actuating forces become necessary in order to cause the brake installation to respond, whereby the driver must actuate the brake pedal with about 50 to 60% of the maximum force to be applied by the driver until the brake pressure build-up starts. This is the consequence, on the one hand, of the fact that owing to the additional annular seals, by means of which the actuating piston of the actuating piston of the brake force servo unit containing the brake valve has to be displaceably sealed off with respect to the housing by means of altogether four additional annular seals, each of which contributes a friction to the friction resistance that is equivalent to several bar, and that--by reason of these friction losses--also relatively strong return springs are required urging the master cylinder pistons into the base positions thereof, which also provide a contribution to the forces equivalent to a pressure of several bar, against which the pistons have to be moved and which in case of a failure of the brake force booster or servo unit act in the sense of the pedal "becoming harder", i.e., a brake pressure can then be built up with an inoperative brake force servo unit only when the driver applies an actuating force corresponding to a minimum brake pressure of about 20 to 30 bar. As the actuating force necessary therefor is quite considerable, the driver, if he has to brake with an inoperative brake force servo unit, may receive the impression that the entire brake installation is inoperative which the driver will sense at least as a threatening situation. However, it may also lead in non-too-rare cases to a panic-like reaction which may become dangerous in a situation which requires a braking.